The present invention relates to optical modulators, and more specifically to a technique for linearizing the output of an external optical intensity modulator.
Recently, there has been a growing interest in the development of analog, amplitude modulated optical communication systems. In comparison with digital systems, analog communication systems provide an efficient use of bandwidth. This is particularly useful in cable television (CATV) transmission system applications, where it is necessary to transmit a large number of video channels through an optical fiber. Compatibility with existing equipment is achieved by using the same signal format for optical transmission that is in use for coaxial cable signal transmission.
In order to transmit an information signal (e.g., a television signal) over an optical fiber, a light beam ("carrier") must be modulated with the information signal. The "electrooptic effect" has been advantageously used to provide modulators for this purpose. For example, electrooptic modulators using miniature guiding structures are known which operate with a low modulating power.
In electrooptic modulators, the electric field induced linear birefringence in an electrooptic material produces a change in the refractive index of the material which, in turn, impresses a phase modulation upon a light beam propagating through the material. The phase modulation is converted into intensity modulation by the addition of polarizers or optical circuitry. Ideally, an electrooptic modulator should have a linear relationship between its output optical power and the applied modulating voltage.
In a "Mach Zehnder" type electrooptic modulator, an optical carrier (laser beam) is split into two paths. At least one path is electrically phase modulated. The two signals are then recombined in an interferometer to provide an intensity modulated carrier. Typically, lithium niobate (LiNbO.sub.3) is used as the electrooptic material. Waveguides in such materials are readily formed by titanium indiffusion.
The output power curve of a Mach Zehnder modulator is nonlinear. Practical analog optical communications systems, however, demand a high linearity. See, for example, W. I. Way, "Subcarrier Multiplexed Lightwave System Design Considerations for Subscriber Loop Applications", J. Lightwave Technol., Vol. 7, pp. 1806-1818 (1989). Modulator nonlinearities cause unacceptable harmonic and intermodulation distortions. When it is necessary to communicate a large number of channels, as in a CATV application, intermodulation distortions ("IMD") can impose serious limitations on the system performance. In principle, the second order IMD can be filtered out if the bandwidth is less than one octave. However, CATV transmission systems operate with bandwidths of many octaves. The third order IMD can only be eliminated by using devices with linear characteristics.
Injection lasers, for example, are not perfectly linear. They can be limited by second order or third order IMD. By using biases well above the threshold and small optical modulation depths, selected injection lasers can barely meet vestigial sideband amplitude modulation CATV system specifications. This limitation is discussed in G. E. Bodeep and T. E. Darcie, "Semiconductor Lasers Versus External Modulators: A Comparison of Nonlinear Distortion for Lightwave Subcarrier CATV Applications", I.E.E.E. Photonics Technol. Lett., Vol. 1, pp. 401-403 (1989).
Electronic precompensating circuits have been proposed to improve the linearity and reduce IMD in laser communication systems. A quasi-linear electrooptic modulator based on a foreshortened directional coupler was proposed in K. T. Koai and P. L. Liu, "Digital and Quasi-Linear Electrooptic Modulators Synthesized from Directional Couplers", IEEE J. Quantum Electron., QE-12, pp. 2191-2194 (1986). Because the modulator proposed in that article uses a short electrode in the directional coupler, a large modulation voltage is required. The resultant inefficiency of such a system is not acceptable for practical CATV signal distribution.
In another prior art system, a Mach Zehnder interferometer with mixed transverse electric ("TE") and transverse magnetic ("TM") polarizations was used to cancel third order IMD. L. M. Johnson and H. V. Roussell, "Reduction of Intermodulation Distortion in Interferometric Optical Modulators", Opt. Lett., Vol. 13, pp. 928-930 (1988). This solution requires a large DC bias and an accurate TE-TM power ratio. In addition, it suffers from low modulation efficiency because a smaller electrooptic term is used. The output power is the sum of two polarization components.
It would be advantageous to provide an optical circuit level compensation technique for linearizing the output of an external optical intensity modulator. It would be further advantageous to provide such a modulator in which IMD distortions are reduced to an acceptably low level. Such apparatus would have particular application in optical fiber CATV distribution systems, wherein a plurality of television channel signals are multiplexed and carried over a single fiber. It would also be advantageous to provide such apparatus that is economical, readily manufacturable, and reliable. The present invention provides such apparatus.